Bulk acoustic wave filter structure with conductive bridge forming electrical loop with an electrode

ABSTRACT

Disclosed is a Bulk Acoustic Wave (BAW) filter structure with a conductive bridge forming an electrical loop with an electrode for reduced electrical losses. In exemplary aspects disclosed herein, the BAW filter structure includes a transducer with electrodes, a piezoelectric layer between the electrodes, and at least one conductive bridge offset from at least a portion of one of the electrodes by an insulating volume. The conductive bridge forms a first electrical loop between a medial end and a distal end of the electrode. Such a configuration reduces electrical resistance, heat resistance, and/or ohmic losses for improved electrical loss of the BAW filter structure.

RELATED APPLICATIONS

This application claims the benefit of provisional patent applicationSer. No. 63/105,390, filed Oct. 26, 2020, the disclosure of which ishereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present invention relates to a Bulk Acoustic Wave (BAW) filterstructure with at least one conductive bridge forming an electrical loopwith an electrode for reduced electrical losses.

BACKGROUND

Acoustic resonators, and particularly Bulk Acoustic Wave (BAW)resonators, are used in many high-frequency communication applications.In particular, BAW resonators are often employed in filter networks thatoperate at frequencies above 1.5 GHz and require a flat passband, haveexceptionally steep filter skirts and squared shoulders at the upper andlower ends of the passband, and provide excellent rejection outside ofthe passband. BAW-based filters also have relatively low insertion loss,tend to decrease in size as the frequency of operation increases, andare relatively stable over wide temperature ranges. As such, BAW-basedfilters are the filter of choice for many 3rd Generation (3G), 4thGeneration (4G), and 5th Generation (5G) wireless devices. Most of thesewireless devices support cellular, wireless fidelity (Wi-Fi), Bluetooth,and/or near field communications on the same wireless device, and assuch, pose extremely challenging filtering demands. While these demandskeep raising the complexity of the wireless devices, there is a constantneed to improve the performance of BAW resonators and BAW-based filtersas well as decrease the cost and size associated therewith.

Electrical loss in a BAW filter (e.g., BAW die, BAW filter die, etc.),or other BAW structure, can negatively affect performance. To meetfiltering requirements in certain applications (e.g., 5G networks), BAWfilters operate at higher frequencies (e.g., greater than 5 GHz), whichmay require thinner electrodes and/or smaller resonator areas. However,reducing electrode thickness may result in increased resistance and/orelectrical loss. Also, reducing resonator areas requires cascadingmultiple resonators in series to handle high power levels, therebyadding more resistance and/or electrical loss. In certain embodiments,materials with high electrical conductivity (e.g., aluminum copper(AlCu)) may be made thicker to reduce electrical losses, but doing somay result in increased acoustic losses since these materials aretypically acoustically lossy and larger thicknesses cause largerfractions of energy (stress/strain) to be contained in these layers.

SUMMARY

Embodiments of the disclosure are directed to a Bulk Acoustic Wave (BAW)filter structure with a conductive bridge forming an electrical loopwith an electrode for reduced electrical losses. In exemplary aspectsdisclosed herein, the BAW filter structure includes a transducer withelectrodes, a piezoelectric layer between the electrodes, and at leastone conductive bridge offset from at least a portion of one of theelectrodes by an insulating volume. The conductive bridge forms a firstelectrical loop between a medial end and a distal end of the electrode.Such a configuration reduces electrical resistance, heat resistance,and/or ohmic losses for reduced electrical loss of the BAW filterstructure.

One embodiment of the disclosure relates to a bulk acoustic wave (BAW)filter structure. The BAW filter structure, including a substrate and atleast one transducer over the substrate. The at least one transducerincludes a first electrode, a second electrode, a piezoelectric layerbetween the first electrode and the second electrode, and a firstconductive bridge offset from at least a portion of the first electrodeby a first insulating volume. The first electrode includes a firstelectrical medial end and a first electrical distal end. The firstconductive bridge is electrically connected to the first electricalmedial end and the first electrical distal end to form a firstelectrical loop between the first electrical medial end and the firstelectrical distal end of the first electrode.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description, serve to explain the principles of thedisclosure.

FIG. 1 illustrates a conventional Bulk Acoustic Wave (BAW) resonator.

FIG. 2 is a graph of the magnitude and phase of impedance over frequencyresponses as a function of frequency for an ideal BAW resonator.

FIGS. 3A-3C are graphs of phase responses for various BAW resonatorconfigurations.

FIG. 4 illustrates a conventional BAW resonator with a border ring.

FIG. 5A is a schematic of a conventional ladder network.

FIGS. 5B and 5C are graphs of a frequency response for BAW resonators inthe conventional ladder network of FIG. 5A and a frequency response forthe conventional ladder network of FIG. 5A.

FIGS. 6A-6E are circuit equivalents for the ladder network of FIG. 5A atthe frequency points 1, 2, 3, 4, and 5, which are identified in FIG. 5C.

FIG. 7A is a cross-sectional side view of a BAW resonator without aconductive bridge.

FIG. 7B is a top partial view of a filter structure without conductivebridges.

FIG. 8A is a cross-sectional side view of a BAW resonator with aconductive bridge.

FIG. 8B is a top partial view of a BAW filter structure with theconductive bridge of FIG. 8A, illustrating reduced power cascadingrelative to the filter structure of FIG. 7B.

FIG. 9 is a graph illustrating electrical loss comparing filters inwhich resonators with and without the conductive bridge are used.

FIG. 10 is a cross-sectional side view of a BAW resonator without topand bottom conductive bridges coupled to reflective layers of a Braggreflector.

FIG. 11 is a cross-sectional side view of a BAW resonator in whichconductive layers of the Brag reflectors form top and bottom conductivebridges.

FIG. 12A is a perspective view of a BAW resonator with conductivebridges extending around a perimeter of electrodes.

FIG. 12B is a perspective view of a layer of the BAW resonator of FIG.12A, illustrating positioning of bridge vias around a perimeter of aninsulating volume.

FIG. 12C is a perspective view of another embodiment of a layer of theBAW resonator of FIG. 12A, illustrating bridge vias around a perimeterand within an interior area of an insulating volume.

FIG. 13A is a cross-sectional side view of a BAW resonator withconductive bridges illustrating positioning of bridge vias within aresonator area.

FIG. 13B is a cross-sectional side view of a BAW resonator withconductive bridges illustrating positioning of one bridge via of eachconductive bridge within a resonator area.

FIG. 14 is a cross-sectional side view of a BAW resonator with a bridgevia extending outside the resonator area, thereby forming a parasiticresonator.

FIG. 15A is a top view of a BAW resonator with apertures to avoidformation of parasitic resonators.

FIG. 15B is a top view of a top electrode of the BAW resonator of FIG.15A.

FIG. 15C is a top view of a bottom electrode of the BAW resonator ofFIG. 15A.

FIG. 15D is a cross-sectional side view of the BAW resonator of FIG. 15Ataken along line D-D.

FIG. 15E is a cross-sectional side view of the BAW resonator of FIG. 15Ataken along line E-E (offset from line D-D).

FIG. 16 is another embodiment of a BAW resonator with a plurality oftabs and apertures to avoid formation of parasitic resonators.

FIG. 17 is a cross-sectional side view of a Coupled Resonator Filter(CRF) without a conductive bridge.

FIG. 18 is a cross-sectional side view of a CRF with a conductivebridge.

FIG. 19 is a cross-sectional side view of a stacked resonator structurewithout a conductive bridge.

FIG. 20 is a cross-sectional side view of a stacked resonator structurewith a conductive bridge.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It should be understood that, although the terms first, second, etc.,may be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It should also be understood that when an element is referred to asbeing “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element, or intervening elements maybe present. In contrast, when an element is referred to as being“directly connected” or “directly coupled” to another element, there areno intervening elements present.

It should be understood that, although the terms “upper,” “lower,”“bottom,” “intermediate,” “middle,” “top,” and the like may be usedherein to describe various elements, these elements should not belimited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed an“upper” element, and, similarly, a second element could be termed an“upper” element depending on the relative orientations of theseelements, without departing from the scope of the present disclosure.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving meanings that are consistent with their meanings in the contextof this specification and the relevant art and will not be interpretedin an idealized or overly formal sense unless expressly so definedherein.

Disclosed herein is a Bulk Acoustic Wave (BAW) filter structure with aconductive bridge forming an electrical loop with an electrode forreduced electrical losses. In exemplary aspects disclosed herein, theBAW filter structure includes a transducer with electrodes, apiezoelectric layer between the electrodes, and at least one conductivebridge offset from at least a portion of one of the electrodes by aninsulating volume. The conductive bridge forms a first electrical loopbetween a medial end and a distal end of the electrode. Such aconfiguration reduces electrical resistance, heat resistance, and/orohmic losses for improved electrical loss of the BAW filter structure.

Prior to delving into the details of these concepts, an overview of BAWresonators and filters that employ BAW resonators is provided. BAWresonators are used in many high-frequency filter applications. Anexemplary BAW resonator 10 is illustrated in FIG. 1. The BAW resonator10 is a solidly mounted resonator (SMR) type BAW resonator 10 andgenerally includes a substrate 12, a reflector 14 mounted over thesubstrate 12, and a transducer 16 mounted over the reflector 14. Thetransducer 16 rests on the reflector 14 and includes a piezoelectriclayer 18, which is sandwiched between a top electrode 20 and a bottomelectrode 22. The top and bottom electrodes 20, 22 may be formed oftungsten (W), molybdenum (Mo), platinum (Pt), or like material, and thepiezoelectric layer 18 may be formed of aluminum nitride (AlN), zincoxide (ZnO) or other appropriate piezoelectric material. Although shownin FIG. 1 as including a single layer, the piezoelectric layer 18, thetop electrode 20, and/or the bottom electrode 22 may include multiplelayers of the same material, multiple layers in which at least twolayers are different materials, or multiple layers in which each layeris a different material.

The BAW resonator 10 is divided into an active region 24 and an outsideregion 26. The active region 24 generally corresponds to the section ofthe BAW resonator 10 where the top and bottom electrodes 20, 22 overlapand also includes the layers below the overlapping top and bottomelectrodes 20, 22. The outside region 26 corresponds to the section ofthe BAW resonator 10 that surrounds the active region 24.

For the BAW resonator 10, applying electrical signals across the topelectrode 20 and the bottom electrode 22 excites acoustic waves in thepiezoelectric layer 18. These acoustic waves primarily propagatevertically. A primary goal in BAW resonator design is to confine thesevertically-propagating acoustic waves in the transducer 16. Acousticwaves traveling upwardly are reflected back into the transducer 16 bythe air-metal boundary at the top surface of the top electrode 20.Acoustic waves traveling downwardly are reflected back into thetransducer 16 by the reflector 14 or by an air cavity, which is providedjust below the transducer in a Film BAW Resonator (FBAR).

The reflector 14 is typically formed by a stack of reflector layers (RL)28A through 28E (referred to generally as reflector layers 28), whichalternate in material composition to produce a significant reflectioncoefficient at the junction of adjacent reflector layers 28. Typically,the reflector layers 28A through 28E alternate between materials havinghigh and low acoustic impedances, such as tungsten (W) and silicondioxide (SiO2). While only five reflector layers 28A through 28E areillustrated in FIG. 1, the number of reflector layers 28 and thestructure of the reflector 14 will vary from one design to another.

The magnitude (Z) and phase (ϕ) of the electrical impedance as afunction of the frequency (GHz) for a relatively ideal BAW resonator 10are provided in FIG. 2. The magnitude (Z) of the electrical impedance isillustrated by the solid line, while the phase (ϕ) of the electricalimpedance is illustrated by the dashed line. A unique feature of the BAWresonator 10 is that it has both a resonance frequency and ananti-resonance frequency. The resonance frequency is typically referredto as the series resonance frequency (fs), and the anti-resonancefrequency is typically referred to as the parallel resonance frequency(fp). The series resonance frequency (fs) occurs when the magnitude ofthe impedance, or reactance, of the BAW resonator 10 approaches zero.The parallel resonance frequency (fp) occurs when the magnitude of theimpedance, or reactance, of the BAW resonator 10 peaks at asignificantly high level. In general, the series resonance frequency(fs) is a function of the thickness of the piezoelectric layer 18 andthe mass of the bottom and top electrodes 20, 22.

For the phase, the BAW resonator 10 acts like an inductance thatprovides a 90° phase shift between the series resonance frequency (fs)and the parallel resonance frequency (fp). In contrast, the BAWresonator 10 acts like a capacitance that provides a −90° phase shiftbelow the series resonance frequency (fs) and above the parallelresonance frequency (fp). The BAW resonator 10 presents a very low,near-zero resistance at the series resonance frequency (fs) and a veryhigh resistance at the parallel resonance frequency (fp). The electricalnature of the BAW resonator 10 lends itself to the realization of a veryhigh-quality factor (Q) inductance over a relatively short range offrequencies, which has proven to be very beneficial in high-frequencyfilter networks, especially those operating at frequencies around 1.8GHz and above.

Unfortunately, the phase (ϕ) curve of FIG. 2 is representative of anideal phase curve. In reality, approaching this ideal is challenging. Atypical phase curve for the BAW resonator 10 of FIG. 1 is illustrated inFIG. 3A. Instead of being a smooth curve, the phase curve of FIG. 3Aincludes ripple below the series resonance frequency (fs), between theseries resonance frequency (fs) and the parallel resonance frequency(fp), and above the parallel resonance frequency (fp). The ripple is theresult of spurious modes, which are caused by spurious resonances thatoccur in corresponding frequencies. While the vast majority of theacoustic waves in the BAW resonator 10 propagate vertically, variousboundary conditions about the transducer 16 result in the propagation oflateral (horizontal) acoustic waves, which are referred to as lateralstanding waves. The presence of these lateral standing waves reduces thepotential quality factor (Q) associated with the BAW resonator 10.

As illustrated in FIG. 4, a border (BO) ring 30 is formed on or withinthe top electrode 20 to suppress certain of the spurious modes. Thespurious modes that are suppressed by the BO ring 30 are those above theseries resonance frequency (fs), as highlighted by circles A and B inthe phase curve of FIG. 3B. Circle A shows a suppression of the ripple,and thus the spurious mode, in the passband of the phase curve, whichresides between the series resonance frequency (fs) and the parallelresonance frequency (fp). Circle B shows suppression of the ripple, andthus the spurious modes, above the parallel resonance frequency (fp).Notably, the spurious mode in the upper shoulder of the passband, whichis just below the parallel resonance frequency (fp), and the spuriousmodes above the passband are suppressed, as evidenced by the smooth orsubstantially ripple free phase curve between the series resonancefrequency (fs) and the parallel resonance frequency (fp) and above theparallel resonance frequency (fp).

The BO ring 30 corresponds to a mass loading of the portion of the topelectrode 20 that extends about the periphery of the active region 24.The BO ring 30 may correspond to a thickened portion of the topelectrode 20 or the application of additional layers of an appropriatematerial over the top electrode 20. The portion of the BAW resonator 10that includes and resides below the BO ring 30 is referred to as a BOregion 32. Accordingly, the BO region 32 corresponds to an outer,perimeter portion of the active region 24 and resides inside the activeregion 24.

While the BO ring 30 is effective at suppressing spurious modes abovethe series resonance frequency (fs), the BO ring 30 has little or noimpact on those spurious modes below the series resonance frequency(fs), as shown in FIG. 3B. A technique referred to as apodization isoften used to suppress the spurious modes that fall below the seriesresonance frequency (fs).

Apodization works to avoid, or at least significantly reduce, anylateral symmetry in the BAW resonator 10, or at least in the transducer16 thereof. The lateral symmetry corresponds to the footprint of thetransducer 16 and avoiding the lateral symmetry corresponds to avoidingsymmetry associated with the sides of the footprint. For example, onemay choose a footprint that corresponds to a pentagon instead of asquare or rectangle. Avoiding symmetry helps reduce the presence oflateral standing waves in the transducer 16. Circle C of FIG. 3Cillustrates the effect of apodization in which the spurious modes belowthe series resonance frequency (fs) are suppressed. Assuming no BO ring30 is provided, one can readily see in FIG. 3C that apodization fails tosuppress those spurious modes above the series resonant frequency (fs).As such, the typical BAW resonator 10 employs both apodization and theBO ring 30.

As noted above, BAW resonators 10 are often used in filter networks thatoperate at high frequencies and require high Q values. A basic laddernetwork 40 is illustrated in FIG. 5A. The ladder network 40 includes twoseries resonators B_(SER) and two shunt resonators B_(SH), which arearranged in a traditional ladder configuration. Typically, the seriesresonators B_(SER) have the same or similar first frequency response,and the shunt resonators B_(SH) have the same or similar secondfrequency response, which is different than the first frequencyresponse, as shown in FIG. 5B. In many applications, the shuntresonators B_(SH) are a detuned version of the series resonatorsB_(SER). As a result, the frequency responses for the series resonatorsB_(SER)and the shunt resonators B_(SH) are generally very similar yetshifted relative to one another such that the parallel resonancefrequency (f_(P,SH)) of the shunt resonators approximates the seriesresonance frequency (f_(S,SER)) of the series resonators B_(SER). Notethat the series resonance frequency (f_(S,SH)) of the shunt resonatorsB_(SH) is less than the series resonance frequency (f_(S,SER)) of theseries resonators B_(SER). The parallel resonance frequency (f_(P,SH))of the shunt resonators B_(SH) is less than the parallel resonancefrequency (f_(P,SER)) of the series resonators B_(SER).

FIG. 5C is associated with FIG. 5B and illustrates the response of theladder network 40. The series resonance frequency (f_(S,SH)) of theshunt resonators B_(SH) corresponds to the low side of the passband'sskirt (phase 2), and the parallel resonance frequency (f_(P,SER)) of theseries resonators B_(SER) corresponds to the high side of the passband'sskirt (phase 4). The substantially aligned series resonance frequency(_(S,SER)) of the series resonators B_(SER) and the parallel resonancefrequency (f_(P,SH)) of the shunt resonators B_(SH) fall within thepassband.

FIGS. 6A through 6E provide circuit equivalents for the five phases ofthe response of the ladder network 40. During the first phase (phase 1,FIGS. 5C, 6A), the ladder network 40 functions to attenuate the inputsignal. As the series resonance frequency (f_(S,SH)) of the shuntresonators B_(SH) is approached, the impedance of the shunt resonatorsB_(SH) drops precipitously, such that the shunt resonators B_(SH)essentially provide a short to ground at the series resonance frequency(f_(S,SH)) of the shunt resonators (phase 2, FIGS. 5C, 6B). At theseries resonance frequency (f_(S,SH)) of the shunt resonators B_(SH)(phase 2), the input signal is essentially blocked from the output ofthe ladder network 40.

Between the series resonance frequency (f_(S,SH)) of the shuntresonators B_(SH) and the parallel resonance frequency (f_(P,SER)) ofthe series resonators B_(SER), which corresponds to the passband, theinput signal is passed to the output with relatively little or noattenuation (phase 3, FIGS. 5C, 6C). Within the passband, the seriesresonators B_(SER) present relatively low impedance, while the shuntresonators B_(SH) present a relatively high impedance, wherein thecombination of the two leads to a flat passband was steep low andhigh-side skirts. As the parallel resonance frequency (f_(P,SER)) of theseries resonators B_(SER) is approached, the impedance of the seriesresonators B_(SER) becomes very high, such that the series resonatorsB_(SER) essentially present themselves as an opening at the parallelresonance frequency (f_(P,SER)) of the series resonators (phase 4, FIGS.5C, 6D). At the parallel resonance frequency (f_(P,SER)) of the seriesresonators B_(SER) (phase 4), the input signal is again essentiallyblocked from the output of the ladder network 40. During the final phase(phase 5, FIGS. 5C, 6E), the ladder network 40 functions to attenuatethe input signal in a similar fashion to that provided in phase 1. Asthe parallel resonance frequency (f_(P,SER)) of the series resonatorsB_(SER) is passed, the impedance of the series resonators B_(SER)decreases, and the impedance of the shunt resonators B_(SH) normalizes.Thus, the ladder network 40 functions to provide a high Q passbandbetween the series resonance frequency (f_(S,SH)) of the shuntresonators B_(SH) and the parallel resonance frequency (f_(P,SER)) ofthe series resonators B_(SER). The ladder network 40 provides extremelyhigh attenuation at both the series resonance frequency (f_(S,SH)) ofthe shunt resonators B_(SH) and the parallel resonance frequency(f_(P,SER)) of the series resonators. The ladder network 40 providesgood attenuation below the series resonance frequency (f_(S,SH)) of theshunt resonators B_(SH) and above the parallel resonance frequency(f_(P,SER)) of the series resonators B_(SER).

Having provided an overview of BAW resonators and filters that employBAW resonators, FIGS. 7A-20 discuss details of a BAW filter structurewith conductive bridge(s).

FIG. 7A is a cross-sectional side view of a BAW resonator 10 without aconductive bridge. As illustrated, current C flows through the topelectrode 20 from a top electrical medial end 41A to a top electricaldistal end 41B, and through the bottom electrode 22 from a bottomelectrical medial end 42A to a bottom electrical distal end 42B. Inparticular, in the top electrode 20, there is an electrical potentialdifference between the top electrical medial end 41A and the topelectrical distal end 41B, resulting in current flow through the topelectrode 20. Similarly, in the bottom electrode 22, there is anelectrical potential difference between the bottom electrical medial end42A and the bottom electrical distal end 42B, resulting in current flowthrough the bottom electrode 22. As the electrical current only flows inone direction through the top electrode 20 and the bottom electrode 22,this results in a greater difference in electrical potential, greaterheat resistance, and/or greater sheet resistance, which may contributeto losses (e.g., ohmic losses) and/or otherwise negatively affectperformance of the BAW resonator 10.

FIG. 7B is a top partial view of a BAW filter structure 44 (alsoreferred to as a filter die, filter die layout, etc.) without conductivebridges. In particular, the BAW filter structure 44 (e.g., BAW ladderfilter) is designed for 5.65 GHz. The BAW filter structure 44 requiressignificant cascading to handle the power requirements at a 5.65 GHzfrequency.

FIG. 8A is a cross-sectional side view of a BAW resonator 10′ with aconductive bridge 46, 48. In particular, FIG. 8A illustrates a BAWresonator 10′ with a top conductive bridge 46 electrically coupled tothe top electrode 20 and a bottom conductive bridge 48 electricallycoupled to the bottom electrode 22. Although the top electrode 20 andthe top conductive bridge 46 are discussed in detail, the discussionbelow also applies to the bottom electrode 22 and the bottom conductivebridge 48.

The top conductive bridge 46 has a lower electrical resistance than thetop electrode 20 and electrically connects opposing ends (top electricalmedial end 41A and top electrical distal end 41B) of the top electrode20. The bottom conductive bridge 48 has a lower electrical resistancethan the bottom electrode 22 and electrically connects opposing ends(bottom electrical medial end 42A and top electrical distal end 42B) ofthe bottom electrode 22. Such a configuration drives lower the potentialdifference between opposing ends of the top electrode 20 and/or thebottom electrode 22, thereby decreasing current through the topelectrode 20 and/or bottom electrode 22. In certain embodiments, thecurrent C flows in opposite directions in different parts of theresonator (and may be zero somewhere in-between opposing ends of the topelectrode 20 and/or the bottom electrode 22). As a result, the topconductive bridge 46 and/or bottom conductive bridge 48 reduceselectrical resistance of the resonator 10′, heat resistance of theresonator 10′, and/or decreases ohmic losses. In certain embodiments,the conductive bridge 46, 48 needs to be arranged to avoid mechanicallyloading the resonator 10′ (e.g., to preserve normal operation of theresonator and prevent additional mechanical losses).

In certain embodiments, the BAW resonator 10′ (may also be referred toas a BAW filter structure) includes a substrate 12 (see, e.g., FIG. 1)and at least one transducer 16 positioned over the substrate. Thetransducer 16 includes a top electrode 20, a bottom electrode 22, and apiezoelectric layer 18 between the top electrode 20 and the bottomelectrode 22. The top electrode 20 includes a top electrical medial end41A and a top electrical distal end 41B. The bottom electrode 22includes a second electrical medial end 42A and a second electricaldistal end 42B.

The BAW resonator 10′ further includes a top conductive bridge 46 offsetfrom at least a portion of the top electrode 20 by a top insulatingvolume 54. The top conductive bridge 46 is electrically connected to thetop electrical medial end 41A and the top electrical distal end 41B toform a top electrical loop between the top electrical medial end 41A andthe top electrical distal end 41B of the top electrode 20. In certainembodiments, at least a portion of the top conductive bridge 46 includesa more conductive material than the top electrode 20. In certainembodiments, all of the top conductive bridge 46 includes a moreconductive material than the top electrode 20. In certain embodiments,the top conductive bridge 46 includes a plurality of materials (e.g.,multiple layers), where a totality of the top conductive bridge 46 ismore conductive than the top electrode 20.

As the top conductive bridge 46 is more conductive than the topelectrode 20, current flows through the top conductive bridge 46 and thetop electrode 20 (from both the top electrical medial end 41A and thetop electrical distal end 41B). Such a configuration reduces theelectrical potential difference between the top electrical medial end41A and the top electrical distal end 41B, thereby resulting in reducedlosses (e.g., ohmic losses).

The top conductive bridge 46 includes a span portion 50, a top medialconductive via 52A, and a top distal conductive via 52B. The top medialconductive via 52A electrically connects the span portion 50 to the topelectrical medial end 41A of the top electrode 20. The top distalconductive via 52B electrically connects the span portion 50 to the topelectrical distal end 41B of the top electrode 20. The top insulatingvolume 54 is positioned between the top medial conductive via 52A andthe top distal conductive via 52B and is positioned between the topelectrode 20 and the span portion 50. In certain embodiments, additionalconductive vias are used to electrically couple the span portion 50 tothe top electrode 20 (e.g., around a periphery of the resonator 10′,anywhere within the inside of the resonator, etc.).

In certain embodiments, the BAW resonator 10′ further includes a bottomconductive bridge 48 offset from at least a portion of the bottomelectrode 22 by a bottom insulating volume 60. The bottom conductivebridge 48 is electrically connected to the bottom electrical medial end42A and the bottom electrical distal end 42B to form a bottom electricalloop between the bottom electrical medial end 42A and the bottomelectrical distal end 42B of the bottom electrode 22. In certainembodiments, at least a portion of the bottom conductive bridge 48includes a more conductive material than the bottom electrode 22. Incertain embodiments, all of the bottom conductive bridge 48 includes amore conductive material than the bottom electrode 22. In certainembodiments, the bottom conductive bridge 48 includes a plurality ofmaterials (e.g., multiple layers), where a totality of the bottomconductive bridge 48 is more conductive than the bottom electrode 22.

As the bottom conductive bridge 48 is more conductive than the bottomelectrode 22, current flows through the bottom conductive bridge 48 andthe bottom electrode 22 (from both the bottom electrical medial end 42Aand the bottom electrical distal end 42B). Such a configuration reducesthe electrical potential difference between the bottom electrical medialend 42A and the bottom electrical distal end 42B, thereby resulting inreduced losses (e.g., ohmic losses).

The bottom conductive bridge 48 includes a span portion 56, a bottommedial conductive via 58A, and a bottom distal conductive via 58B. Thebottom medial conductive via 58A electrically connects the span portion56 to the bottom electrical medial end 42A of the bottom electrode 22.The bottom distal conductive via 58B electrically connects the spanportion 56 to the bottom electrical distal end 42B of the bottomelectrode 22. The bottom insulating volume 60 is positioned between thebottom medial conductive via 58A and the bottom distal conductive via58B and is positioned between the bottom electrode 22 and the spanportion 56. In certain embodiments, additional conductive vias are usedto electrically couple the span portion 56 to the bottom electrode 22(e.g., around a periphery of the resonator 10′).

In certain embodiments, the BAW resonator 10′ includes the topconductive bridge 46 only (not the second conductive bridge 48). Inother embodiments, the BAW resonator 10′ includes the bottom conductivebridge 48 only (not the first conductive bridge 46).

In certain embodiments, the top insulating volume 54 and/or the bottominsulating volume 60 includes an air cavity. The air cavity avoidsmechanically loading the resonator. In certain embodiments, topinsulating volume 54 and/or the bottom insulating volume 60 includes asolid material (e.g., at least a portion of a Bragg reflector). Incertain embodiments, the top insulating volume 54 includes at least aportion of a top Bragg reflector, and the bottom insulating volume 60includes at least a portion of a bottom Bragg reflector.

FIG. 8B is a top partial view of a BAW filter structure 44′ (e.g., BAWladder filter) with the conductive bridges 46, 48 of FIG. 8A. Inparticular, the BAW filter structure 44′ is designed for 6.5 GHz. TheBAW filter structure 44′ requires cascading to handle the powerrequirements at a 6.5 GHz frequency. However, the BAW filter structure44′ with the conductive bridges 46, 48 reduces needed cascading, even athigher frequencies, such as to improve power dissipation density,thereby resulting in reduced insertion losses.

FIG. 9 is a graph 62 illustrating electrical loss comparing resonatorswith and without the conductive bridge. In particular, the graph 62compares results for BAW filter structures 44, 44′ (BAW ladder filter)designed for 6.5 GHz with and without conductive bridges 46, 48 (e.g.,when metal reflector layers of Bragg reflectors are electrically coupledto the top electrodes and bottom electrodes). When conductive bridges46, 48 are used, insertion loss improved by 0.5 dB. This improvement maybe higher if higher rejections and/or higher power handling is required(where more stages and/or more cascading are necessary).

FIG. 10 is a cross-sectional side view of a BAW resonator 63 without topand bottom conductive bridges 46, 48 electrically coupled to reflectivelayers 28A(1)-28E(2) of a Bragg reflector 14(1), 14(2). The BAWresonator 63 includes a top reflector 14(1) proximate the top electrode20 and a bottom reflector 14(2) proximate the bottom electrode 22. Assimilarly discussed above, in such a configuration, the electricalcurrent C flows from electrical medial ends 41A, 42A to the electricaldistal ends 41B, 42B through the top and bottom electrodes 20, 22.

FIG. 11 is a cross-sectional side view of a BAW resonator 63′ with topand bottom conductive bridges 46, 48. As discussed above in FIG. 10, theBAW resonator 63′ includes a top Bragg reflector 14(1) proximate the topelectrode 20 and a bottom Bragg reflector 14(2) proximate the bottomelectrode 22.

In certain embodiments, the top Bragg reflector 14(1) includes reflectorlayers 28A(1)-28E(1). While only five reflector layers 28A through 28Eare illustrated, the number of reflector layers 28 and the structure ofthe reflector 14 may vary. Although the top electrode 20 and the topBragg reflector 14(1) are discussed in detail below, the discussionbelow also applies to the bottom electrode 22 and the bottom Braggreflector 14(2).

Reflector layer 28A(1) includes an electrically insulating material.Reflector layers 28B(1)-28E(1) include an electrically conductivematerial (e.g., highly conductive metallic materials). In this way, asreflector layers 286(1)-28E(1) are electrically connected to each other,together reflector layers 286(1)-28E(1) form a span portion 50 of thetop conductive bridge 46. A top medial conductive via 52A and a topdistal conductive via 52B electrically connect the top electrode 20 toreflector layer 28B(1). Accordingly, current flows through the topelectrode 20 and separately flows through the top medial conductive via52A through reflector layers 28B(1)-28E(1) and through the top distalconductive via 52B.

As noted above, Bragg reflectors 14(1), 14(2) typically alternatebetween materials having high and low acoustic impedances. Reflectorlayers 28B(2) similarly alternate between metallic materials having highand low acoustic impedances. In certain embodiments, Reflector layer28A(1) includes SiO2, reflector layers 28B(1) and 28D(1) include analuminum (e.g., AlCu), and reflector layers 28C(1) and 28E(1) includesW. As part of a Bragg reflector, reflector layers 28B(1) and 28D(1)exhibit relatively small stress/strain, and therefore acoustic losses inthese layers are reduced.

FIGS. 12A-12B are views of a BAW resonator 63″ with conductive vias52A-52B of conductive via layers 52(1), 52(2) of conductive bridges 46,48 extending around a perimeter of the top and bottom electrodes 20, 22.In particular, the span portion 50 of the top conductive bridge 46 has afootprint or surface area of at least 60% (e.g., at least 70%, at least80%, at least 90%, at least 95%, at least 99%, 100%, etc.) of that ofthe top electrode 20. The top conductive bridge 46 further includes topside conductive vias 52C, 52D extending between the top medialconductive via 52A and the top distal conductive via 52B to enclose thetop insulating volume 54 between the span portion 50 and the topelectrode 20. It is noted that a similar configuration is also appliedto the bottom conductive bridge 48 and bottom electrode 22.

Conductive bridge vias 52A-52D are positioned at pairs of opposing endsof the top electrode 20 (e.g., at each of four sides of a square). Insuch a configuration, current flows from all sides toward an approximatecenter, thereby forming a two-dimensional electrical gradient (i.e.,current flow is about zero at a center of the square plane). In otherembodiments, bridge vias 52A-52D are positioned at opposing ends 41A,41B of the top electrode 20 (i.e., at two opposing sides of a square).In such a configuration, current flows from one side toward the otherside, forming a one-dimensional electrical gradient (i.e., current flowis about zero along an approximate centerline of the square plane).

As an example of performance, in one embodiment, at 6.3 GHz, a BAWresonator 63 without a conductive bridge 46, 48 results in an inputresistance of 0.49 Ohm, the BAW resonator 63′ with conductive bridges46, 48 connected to top and bottom electrodes 20, 22 at two sidesresults in an input resistance of 0.164 Ohm, and the BAW resonator 63″with conductive bridges 46, 48 connected to top and bottom electrodes atfour sides results in an input resistance of 0.0947 Ohm. Accordingly,the conductive bridge 46, 48 clearly improves Ohmic losses in theelectrodes 20, 22.

FIG. 12C is a perspective view of another embodiment of a layer 52(1)′of the BAW resonator 63″ of FIG. 12A, illustrating peripheral bridgevias 52A-52D around a perimeter and internal bridge vias 52E within aninterior area of an insulating volume of the peripheral bridge vias52A-52D. Any number, pattern, shape and/or size of internal bridge vias52E may be used.

FIG. 13A is a cross-sectional side view of a BAW resonator 64 withconductive bridges 46, 48 illustrating positioning of conductive bridgevias 52A, 52B, 58A, 58B within an active region 24. In certainembodiments, the conductive bridge vias 52A, 52B, 58A, 58B may beincorporated into a border ring 30 (as discussed above in FIG. 4). Anadvantage of such a configuration is that no further modifications needto be made structurally to the BAW resonator 64 to accommodate theconductive bridge vias 52A, 52B, 58A, 58B. In other words, theconductive bridge vias 52A, 52B, 58A, 58B do not alter the intendedactive region 24. However, the BAW resonator 64 has to be designed tofactor in the effect the conductive vias 52A, 52B, 58A, 58B have on theoperation of the border ring 30.

In certain embodiments, the top electrode 20 includes a top medial viaportion 66A within the active region 24 and aligned with the top medialvia 52A, and a top distal via portion 66B within the active region 24and aligned with the top distal via 52B. Similarly, in certainembodiments, the bottom electrode 22 includes a bottom medial viaportion 68A within the active region 24 and aligned with the bottommedial via 58A, and a bottom distal via portion 68B within the activeregion 24 and aligned with the bottom distal via 58B.

FIG. 13B is a cross-sectional side view of a BAW resonator 62′ withconductive bridges 46, 48 illustrating positioning of one conductivebridge via 52B, 58A of each conductive bridge 46, 48 within an activeregion 24. As noted above, an advantage of such a configuration isavoidance complications involving electrical overlap of conductivebridge vias 52A, 52B, 58A, 58B outside the active region 24. Inparticular, the top electrical medial end 41A of the top electrode 20 isoutside the active region 24, while the bottom electrical distal end 42Bof the bottom electrode is outside the active region 24. Accordingly,there is no overlap of the top electrode 20, and the bottom electrode 22outside of the active region 24. Such positioning means that any effectof the conductive bridge vias 52A, 52B, 58A, 58B may be asymmetricalacross the active region 24, but may also reduce the effect of theconductive vias on the BAW resonator 64′ across the active region.

In certain embodiments, the top electrode 20 includes a top medial viaportion 66A external to the active region 24 and aligned with the topmedial via 52A, and a top distal via portion 66B within the activeregion 24 and aligned with the top distal via 52B. Similarly, in certainembodiments, the bottom electrode 22 includes a bottom medial viaportion 68A external to the active region 24 and aligned with the bottommedial via 58A, and a bottom distal via portion 68B within the activeregion 24 and aligned with the bottom distal via 58B.

FIG. 14 is a cross-sectional side view of a BAW resonator 70 withconductive bridges 46, 48 extending outside the resonator area formingparasitic resonator 71(1), 71(2). In certain embodiments, the conductivebridge vias 52A, 52B, 58A, 58B are positioned outside the active region24. In such a configuration, the conductive material of the topelectrode 20 and/or bottom electrode 22 are uniform across the activeregion 24 of the BAW resonator 70. However, this results in an overlapof the top electrode 20 and the bottom electrode 22 outside of theintended active region 24. As a result, parasitic resonators 71(1),71(2) may be formed due to this overlap, which may adversely affectresonator coupling and may affect its Q-factor as well.

FIGS. 15A-15E illustrate a BAW resonator 72 with apertures to avoidformation of parasitic resonators 71(1), 71(2) (see FIG. 14). The BAWresonator 72 has the top conductive bridge 46, and the bottom conductivebridge 48 in non-overlapping areas outside the active region 24 to avoidparasitic resonators.

Referring to FIGS. 15A and 15B, the BAW resonator 72 includes a topelectrode 20 with a top active region portion 74, a top medial portion76 external to the top active region portion 74, a top medial viaportion 66A(1), 66A(2) external to the top active region portion 74 andaligned with the top medial via 52A, and a top distal via portion 66Bexternal to the top active region portion 74 and aligned with the topdistal via 52B. The top medial portion 76 does not overlap withconductive vias 52A, 52B, or a span portion 50 of the top conductivebridge 46. The top medial via portion 66A(1), 66A(2) overlaps with topmedial conductive vias 52A of the top conductive bridge 46. The topdistal via portion 66B overlaps with top distal conductive vias 52B ofthe top conductive bridge 46. The span portion 50 overlaps with at leasta portion (or the entirety of) the top active region portion 74, the topmedial via portion 66A(1), 66A(2), and the top distal via portion 66B.In certain embodiments, the top medial via portion 66A(1), 66A(2) is agreater size than the top distal via portion 66B. Further, the topmedial via portion 66A(1) includes a plurality of top medial viaportions 66A(1), 66A(2) separated from each other by a single gap 78.The top distal via portion 66B includes a single top distal via portion66B.

Referring to FIGS. 15A and 15C, the BAW resonator 72 includes a bottomelectrode 22 with a bottom active region portion 80, a bottom medialportion 82 external to the bottom active region portion 80, a bottommedial via portion 66B external to the bottom active region portion 80and aligned with the bottom medial via 58A, and a top distal via portion68B(1), 68B(2) external to the bottom active region portion 80 andaligned with the bottom distal via portion 68B(1), 68B(2). The bottommedial portion 82 does not overlap with conductive vias 58A, 58B or aspan portion 56 of the bottom conductive bridge 48. The bottom medialvia portion 66B overlaps with bottom medial conductive vias 58A of thebottom conductive bridge 48. The bottom distal via portion 68B(1),68B(2) overlaps with bottom distal conductive vias 58B of the bottomconductive bridge 48. The span portion 56 overlaps with at least aportion (or the entirety of) the bottom active region portion 80, thebottom medial via portion 66B, and the bottom distal via portion 68B(1),68B(2). In certain embodiments, the bottom medial via portion 66B is asmaller size than the bottom distal via portion 68B(1), 68B(2). Further,the bottom distal via potion 68B(1), 68B(2) includes a plurality ofbottom medial via portions 68B(1), 68B(2) separated from each other by asingle gap 84. The bottom medial via portion 66B includes a singlebottom medial via portion 66B.

The active region portion 74 of the top electrode 20 overlaps with theactive region portion 80 of the bottom electrode 22 to form an activeregion 24 of the BAW resonator 72. A top tab 86 of the top electrode 20including the top distal via portion 66B is positioned in the bottom gap84 of the bottom electrode 22, and a bottom tab 88 of the bottomelectrode 22 including the bottom medial via portion 68A is positionedin the top gap 78 of the top electrode 20. In other words, the singletop distal via portion 66B vertically aligns with the bottom gap 84, andthe single bottom medial via portion 68A vertically aligns with the topgap 78.

Referring to FIGS. 15A-15E, the top medial via portion 66A(1), 66A(2),top distal via portion 66B, bottom medial via portion 68A, and bottomdistal via portion 68B(1), 68B(2) do not overlap with each other,thereby avoiding parasitic resonators. In particular, FIG. 15D is across-sectional side view of the BAW resonator 72 of FIG. 15A takenalong line D-D. FIG. 15E is a cross-sectional side view of the BAWresonator 72 of FIG. 15A taken along line E-E (offset from line D-D).

Accordingly, the conductive vias 52A, 52B of the top conductive bridge46 connected to the top electrode 20 are positioned outside the activeregion 24 and do not overlap with the bottom electrode 22. Similarly,the conductive vias 58A, 58B of the bottom conductive bridge 48connected to the bottom electrode 22 are positioned outside the activeregion 24 and do not overlap with the top electrode 20.

It is noted that the size of the single top distal via portion 66B,single bottom medial via portion 68A, bottom gap 84, and/or top gap 78may be larger or smaller. In particular, larger sizes increase surfacearea of the electrical contact portion, reduce heat resistance, and/ordecrease current flowing through top electrode 20 and bottom electrode.

FIG. 16 is another embodiment of a BAW resonator 72′ with gaps 78, 84 toavoid formation of parasitic resonators. The top medial via portion 66Aincludes a plurality of top medial via portions 66A separated from eachother by a plurality of gaps 78. A plurality of top medial tabs 90includes the top medial via portions 66A and a plurality of top distaltabs 86 includes the top distal via portions 66B. The plurality of tabs86, 90 are positioned around a periphery of the active region portion 74(as in FIGS. 15A-15E above) of the top electrode 20. At least a portionof the plurality of medial tabs 90 are electrically connected to amedial portion 76 of the top electrode 20.

Similarly, the bottom medial via portion 68A includes a plurality ofbottom distal via portions 68B separated from each other by a pluralityof gaps 84. A plurality of bottom medial tabs 88 includes the bottommedial via portions 68A, and a plurality of bottom distal tabs 92include the bottom distal via portions 68B. The plurality of tabs 86,88, 90, and 92 are positioned around a periphery of the active regionportion 80 (as in FIGS. 15A-15E above) of the bottom electrode 22. Atleast a portion of the plurality of distal tabs 92 are electricallyconnected to a distal portion 82 of the bottom electrode 22.

The tabs 86-92 of the top electrode 20 and the bottom electrode 22 arepositioned around a periphery of the active region 24 of the BAWresonator 72′. At least a portion of the top distal via portions 66Bvertically aligns with one of the plurality of gaps 84, and at least aportion of the bottom medial via portions 68A vertically aligns with oneof the plurality of gaps 78.

It is noted that the size of the top distal via portion 66B, bottommedial via portion 68A, bottom gaps 84, and/or top gaps 78 may be largeror smaller. Further, the plurality of contacts may equalize potential inthe resonator more as the contact portions are positioned around theentire periphery (i.e., flow is more centralized and less concentratedfrom one side to the other).

FIG. 17 is a cross-sectional side view of a Coupled Resonator Filter(CRF) 94 without a conductive bridge. The CRF 94 includes fourresonators 10(1)-10(4), each with a piezoelectric layer 18(1), 18(2), atop electrode 20A(1), 20A(2), 20B, and a bottom electrode 22A(1),22A(2), 22B. Electrical energy enters through the first resonator 10(1)and leaves through the fourth resonator 10(4). The first resonator 10(1)is vertically positioned above the second resonator 10(2), and thefourth resonator 10(4) is vertically positioned above the thirdresonator 10(3). The first resonator 10(1) is horizontally positionedadjacent to the fourth resonator 10(4), and the second resonator 10(2)is horizontally positioned adjacent to the third resonator 10(3).Further, the top electrodes 20A(1), 20A(2), and bottom electrodes22A(1), 22A(2) of the first and fourth resonators 10(1), 10(4) areelectrically isolated from one another. The top and bottom electrodes20B, 22B of the second and third resonators 10(2), 10(3) areelectrically coupled to one another. Between the top resonators 10(1),10(4) and the bottom resonators 10(2), 10(3) is coupler 95, including aplurality of coupling layers 96A, 96B, 96C. Accordingly, the firstresonator 10(1) and the second resonator 10(2) are acoustically coupledto one another, the second resonator 10(2) and third resonator 10(3) areelectrically coupled to one another, and the third resonator 10(3) andfourth resonator 10(4) are acoustically coupled to one another.

FIG. 18 is a cross-sectional side view of a CRF 94′ with a conductivebridge 46. In this configuration, the conductive bridge 46 includesconductive vias 52A, 52B, 52C extending from a metal coupling layer 96Bof the coupler to the top electrode 20B of the second and thirdresonators 10(1), 10(2). The conductive vias 52A, 52B, 52C arepositioned at a medial end 41A of the second resonator 10(2), betweenthe second resonator 10(2) and the third resonator 10(3) (the distal endof the second resonator 10(2) and the medial end of the third resonator10(3)), and at a distal end 41B of the third resonator 10(3). Althoughonly one conductive bridge 46 for the top electrode 20B of the secondand third resonators 10(2), 10(3) is shown, it is noted that conductivebridges 46 could be applied to each of the four resonators 10(1)-10(4),as similarly discussed above.

In other words, the BAW filter structure includes a coupled-resonatorfilter (CRF) structure 94′. The at least one transducer 16 includes abottom plurality of horizontally adjacent transducers 16(2), 16(3). Incertain embodiments, each of the bottom plurality of horizontallyadjacent transducers 16(2), 16(3) includes a conductive bridge 46. Incertain embodiments, a top plurality of transducers 16(1), 16(4)positioned above the bottom plurality of horizontally adjacenttransducers 16(2), 16(3) with the conductive bridge 46 positionedbetween the bottom plurality of horizontally adjacent transducers 16(2),16(3) and the top plurality of horizontally adjacent transducers 16(1),16(4)

FIG. 19 is a cross-sectional side view of a stacked resonator structure96 without a conductive bridge. In a stacked configuration, the topresonator 10(1) is vertically positioned above the bottom resonator10(2). A decoupler 98, including a decoupling layer 100, is positionedbetween the top resonator 10(1) and the bottom resonator 10(2) toacoustically decouple the top resonator 10(1) from the bottom resonator10(2). Each of the top resonator 10(1) and bottom resonator 10(2)includes a top reflector 14(1) with a plurality of reflector layers28A(1)-28E(1) and a bottom reflector 14(2) with a plurality of reflectorlayers 28A(1)-28E(2).

FIG. 20 is a cross-sectional side view of a stacked resonator structure96′ with a conductive bridge 46. Similar to other embodiments discussedabove, the top electrode 20 of the top resonator 10(1) is electricallycoupled to a metal layer of the Bragg reflector 14(1) by conductive vias52A, 52B, and the bottom electrode 22 of the top resonator 10(1) iselectrically coupled to a metal layer of the Bragg reflector 14(2) byconductive vias 58A, 58B. In other words, the bottom electrode 22 of thetop resonator 10(1) is electrically coupled to a metal layer of thedecoupler 100. In other words, the BAW filter structure 96′ comprises astacked resonator filter structure, including a plurality of verticallyadjacent transducers 16(1), 16(2). Each of the plurality of verticallyadjacent transducers 16(1), 16(2) includes a top conductive bridge 46and/or a bottom conductive bridge 48.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A bulk acoustic wave (BAW) filter structure,comprising: a substrate; at least one transducer over the substrate, theat least one transducer comprising: a first electrode comprising a firstelectrical medial end and a first electrical distal end, a secondelectrode, a piezoelectric layer between the first electrode and thesecond electrode, and a first conductive bridge offset from at least aportion of the first electrode by a first insulating volume, wherein thefirst conductive bridge is electrically connected to the firstelectrical medial end and the first electrical distal end to form afirst electrical loop between the first electrical medial end and thefirst electrical distal end of the first electrode.
 2. The BAW filterstructure of claim 1, wherein the first electrode is a top electrode,and the second electrode is a bottom electrode.
 3. The BAW filterstructure of claim 1, wherein the first electrode is a bottom electrode,and the second electrode is a top electrode.
 4. The BAW filter structureof claim 1, wherein the first conductive bridge comprises: a spanportion, a first medial conductive via electrically connecting the spanportion to the first electrical medial end of the first electrode, and afirst distal conductive via electrically connecting the span portion tothe first electrical distal end of the first electrode.
 5. The BAWfilter structure of claim 4, wherein the first conductive bridge furthercomprises first side conductive vias extending between the first medialconductive via and the first distal conductive via to enclose the firstinsulating volume between the span portion and the first electrode. 6.The BAW filter structure of claim 1, wherein the first conductive bridgecomprises: a span portion, peripheral conductive vias enclosing aninsulating volume; and internal conductive vias within the firstinsulating volume enclosed by the peripheral conductive vias.
 7. The BAWfilter structure of claim 1, wherein the first insulating volumecomprises an air cavity.
 8. The BAW filter structure of claim 1, whereinthe first insulating volume comprises at least a portion of a firstBragg reflector.
 9. The BAW filter structure of claim 1, furthercomprising a first Bragg reflector positioned proximate the firstelectrode, the first Bragg reflector comprising a proximate metalliclayer; wherein the first conductive bridge comprises metallic viasextending between the first electrode and the proximate metallic layerof the first Bragg reflector.
 10. The BAW filter structure of claim 1,wherein the first electrode comprises a first active region portion, afirst medial via portion external to the first active region portion andaligned with the first medial via, and a first distal via portion withinthe first active region portion and aligned with the first distal via.11. The BAW filter structure of claim 1, wherein the first electrodecomprises a first active region portion, a first medial via portionexternal to the first active region portion and aligned with the firstmedial via, and a first distal via portion external to the first activeregion portion and aligned with the first distal via, the first medialvia portion being a greater size than the first distal via portion. 12.The BAW filter structure of claim 1, wherein the first medial viaportion comprises a plurality of first medial via portions separatedfrom each other by at least one first gap.
 13. The BAW filter structureof claim 1, wherein the first distal via portion comprises a singlefirst distal via portion.
 14. The BAW filter structure of claim 1,wherein the first distal via portion comprises a plurality of firstdistal via portions separated from each other by a gap.
 15. The BAWfilter structure of claim 1, further comprising a second conductivebridge offset from at least a portion of the second electrode by asecond insulating volume, wherein the second electrode comprises asecond electrical medial end and a second electrical distal end, whereinthe second conductive bridge is electrically connected to the secondelectrical medial end and the second electrical distal end to form asecond electrical loop between the second electrical medial end and thesecond electrical distal end of the second electrode.
 16. The BAW filterstructure of claim 15, wherein the first insulating volume comprises atleast a portion of a first Bragg reflector, and the second insulatingvolume comprises at least a portion of a second Bragg reflector.
 17. TheBAW filter structure of claim 15, wherein the first electrode comprisesa first active region portion, a first medial via portion aligned with afirst medial via, and a first distal via portion aligned with a firstdistal via, the first medial via portion being a greater size than thefirst distal via portion; wherein the second electrode comprises asecond active region portion, a second medial via portion aligned with asecond medial via, and a second distal via portion aligned with a seconddistal via, the second medial via portion being a greater size than thesecond distal via portion; wherein the first distal via portioncomprises a single first distal via portion; wherein the first medialvia portion comprises a plurality of first medial via portions separatedfrom each other by a first gap; wherein the second distal via portioncomprises a single second distal via portion; wherein the second medialvia portion comprises a plurality of second medial via portionsseparated from each other by a second gap; and wherein the single firstdistal via portion vertically aligns with the second gap, and the singlesecond distal via portion vertically aligns with the first gap.
 18. TheBAW filter structure of claim 15, wherein the first electrode comprisesa first active region portion, a first medial via portion aligned withthe first medial via, and a first distal via portion aligned with thefirst distal via, the first medial via portion being a greater size thanthe first distal via portion; wherein the second electrode comprises asecond active region portion, a second medial via portion aligned withthe second medial via, and a second distal via portion aligned with thesecond distal via, the second medial via portion being a greater sizethan the second distal via portion; wherein the first distal via portioncomprises a plurality of first distal via portions; wherein the firstmedial via portion comprises a plurality of first medial via portionsseparated from each other by a first plurality of gaps; wherein thesecond distal via portion comprises a single second distal via portion;wherein the second medial via portion comprises a plurality of secondmedial via portions separated from each other by a second plurality ofgaps; and wherein each of the first distal via portions verticallyaligns with one of the second plurality of gaps, and each of the seconddistal via portions vertically aligns with one of the second pluralityof gaps.
 19. The BAW filter structure of claim 1, wherein the BAW filterstructure comprises a coupled-resonator filter (CRF) structure; andwherein the at least one transducer includes a first plurality ofhorizontally adjacent transducers, each of the first plurality ofhorizontally adjacent transducers including the first conductive bridge.20. The BAW filter structure of claim 1, further comprising a secondplurality of horizontally adjacent transducers positioned above thefirst plurality of horizontally adjacent transducers with the firstconductive bridge positioned between the first plurality of horizontallyadjacent transducers and the second plurality of horizontally adjacenttransducers.
 21. The BAW filter structure of claim 1, wherein the BAWfilter structure comprises a stacked resonator filter structure; andwherein the at least one transducer includes a plurality of verticallyadjacent transducers, each of at least a portion of the plurality ofvertically adjacent transducers including the first conductive bridge.